Building materials hardened by autoclave treatment are increasingly being used in the building materials industry. Sandlime bricks and fiber reinforced concretes are two examples of these. After shaping, these materials are hardened under pressure, usually up to 20 bar, at temperatures up to 250.degree. C. The autoclave conditions frequently used are 205.degree. C., 16 bar, 4 hours.
Sandlime bricks colored with Fe.sub.3 O.sub.4 black will discolor and become red under these conditions. Manganese-containing iron oxides have also been employed as heat-stable pigments for some time. These are mixed phases with a bixbyite (Mn.sub.2 O.sub.3) or corundum structure prepared by annealing at high temperatures.
Because of the calcination process needed for the preparation of these pigments, they are weak in tinctorial strength and difficult to disperse in comparison with precipitated black Fe.sub.3 O.sub.4 pigments. Although Mn-containing mixed phase pigments, such as e.g. Bayferrox 303T.RTM. (commercial product from Bayer AG) retain their color shade, they are weak in tinctorial strength. Thus, in contrast to an Fe.sub.3 O.sub.4 pigment of high tinctorial strength, e.g. Bayferrox 330.RTM., a tinctorial strength of only 35% is reached. This means that about three times the amount of Bayferrox 330T.RTM. pigment has to be employed in order to achieve the same depth of color as with Bayferrox 330.RTM..
The object of this invention is thus to provide a black pigment which does not have the disadvantages described.